ALPHA SEX (Alpha Sextantis). In the pantheon of modern
constellations, one navigational/positional instrument (Octans, the
Octant) finds the way in the southern sky, while another, Quadrans
(the Quadrant) seeks the stars in the northern, while yet another,
Sextans (the Sextant) falls in the
middle, straddling the celestial equator. Only two of these,
however, remain, Quadrans long ago dropped from the official roll
call. Neither is very bright. The luminary of Sextans, Alpha
Sextantis, just barely escapes fifth magnitude (4.49) by a mere
hundredth of a division. Physically there is not too much special
about this class A (A0) giant. With a surface temperature of 9900
Kelvin, it shines from a distance of 285 light years
with a luminosity 122 times that of the Sun, its
mass 3 times solar. While spectroscopically a "giant" (with a
radius only 3.8 times that of the Sun), this
300 million year old star is simply nearing the end of its
hydrogen-fusing lifetime. It will within a short time-span of only
60 million years become a true and much brighter orange giant, its
hydrogen core expired to helium. The lower limit to the equatorial
rotation velocity is but 7 km/s. Since most stars of this class
rotate more rapidly, and since low rotators tend to be chemically
peculiar (which the star is not), Alpha Sextantis's rotation pole
probably points towards Earth. The star's claim to any sort of
greatness lies in its position. It is almost exactly south of Regulus in Leo,
shifted by a mere 0.4 minutes of arc (0.007 of a degree) to the
west. More interesting, it is one of the sky's informal "equator
stars," currently lying less than a quarter of a degree south of
the celestial equator. The action of the Moon and Sun on the
Earth's equatorial bulge slowly causes the axis of the Earth to
wobble over a 26,000 year period, which in turn causes changes in
the pole stars and in stellar positions. In 1900, Alpha Sextantis
was 7 minutes of arc NORTH of the equator, rather than south of it.
It crossed over from one hemisphere to the other in December of
1923 (the celestial equator actually shifting, not the star).